A. Field of the Invention
The present invention relates to a galvanized steel sheet with a zero-spangle, a galvanizing process therefor, and a composition of a molten zinc bath therefor. More specifically, this invention is directed to a method for preventing the flaking off or peeling of the zinc coating of a hot-dip galvanized steel sheet (referred to galvanized steel sheet hereinafter) which is apt to occur when the galvanized steel sheet is subjected to working after it has been stored indoor over a long period as the result of transgranular fracture caused by corrosion and the propagation of slow cracks in the galvanized coating during the long indoor storage period.
The term "Zero-spangle" used throughout in this specification refers to a minimum spangle which is much finer than a large spangle.
B. Description of the Prior Art
Galvanized steel sheet is one of the most extensively manufactured coated steel sheets at the present time and it is in strong demand in many fields of industry for automobiles, building, electric home appliances, and the like. Recently, the advent of complicated and diversified uses and remarkable progress in both painting and film adhesion techniques has, together with the trend toward complicated forms of fabrication, given rise to an urgent need for a surface treated steel sheet of a much higher quality than has been available up to now.
On the other hand, it is recently often noted that when a painted galvanized steel sheet or a galvanized steel sheet coated with vinyl chloride is used under conditions where it is exposed to the weather or to a high temperature humid atmosphere, there occurs blistering or peeling of the coated layer together with the coating in the form of spots or flakes due to intergranular corrosion. This greatly reduces the commercial value of the galvanized steel sheet as a surface-treated steel sheet product.
As a countermeasure to the above described defect, it has already been proposed that instead of a molten zinc coating, there be used a method of adding 0.02-0.15 wt% Sb (wt stands for weight) to an alloy of Zn and 0.2-17 wt% Al. As an example in which Sb is added to a molten zinc bath in order to improve both phosphate treatment and paint adherence of a high Pb galvanized steel sheet, there is known a method wherein 0.01-0.50 wt% Sb is added to the hot-dip zinc bath.
In addition to the flaking of a coating layer resulting from grain boundary corrosion in a high temperature humid atmosphere, the present inventors have seen the coating layer of a galvanized steel sheet flake off when exposed to an ordinary indoor atmosphere. They carried out research to determine the cause of the phenomenon. Their findings are as follows:
The coating layers of both painted galvanized steel sheet and galvanized steel sheet coated with vinyl chloride will flake off after an indoor aging period of more than one year from the time of galvanizing, even when not exposed to a high temperature humid atmosphere (95.degree. C., RH&gt;98%), at the time the sheet is subjected to working.
As regards the flaking-off of a coating layer as described above, it is seen that the finer the spangle on the surface of a galvanized steel sheet the more the flaking of the coating layer tends to occur. The commonly used coating layer having a large spangle resists flaking off.
From the above findings, the inventors have concluded that the chief cause for flaking off of a coating layer is the composition of the galvanizing bath or a difference in the nature of the coating layer. They, therefore conducted a study on the cause and obtained the following results:
It was concluded that the occurrence of peeling of a coating layer in the case where a galvanized steel sheet is subjected to working after being left to stand for a long period in a weak corrosive environment is due to transgranular fracture caused by the slow enlargement and propagation of cracks which occur at the time of production of a minimum spangle by rapid cooling, with corrosion being another contributing factor. On the other hand, in a high-temperature humid atmosphere, flaking of the coating layer accompanies the progress of grain boundary corrosion.
A crack which developed in a coating layer with a zero-spangle after the galvanized steel sheet had been stored for a long period was analyzed by an X-ray microanalyzer (referred to as an EMX hereinafter). From the analysis it was concluded that both Al and O had been concentrated to produce an anchor effect.
On analyzing the elements which remained at the side of the steel sheet in the flaking region by EMX, it was found that there was little Zn and much Fe and Al. Hence, it is seen that flaking of a coating layer finally takes place in the vicinity of a Fe-Al-Zn ternary alloy.
In other words, one of the causes in connection with flaking of a coating layer which occurs concentratedly in zero-spangle galvanized steel sheet lies in the occurrence of a crack in the coating layer resulting from the rapid cooling applied to the surface of the coating layer when it is in a semi-molten state.
The second cause is that when the galvanized steel sheet stands in storage for a long time, water adsorbed on the surface of the coating finds its way through the cracks into interior regions of the coating, selective corrosion of segregated Al in the coating layer occurs, and the corrosive atmosphere of the crack is increased in alkalinity by the corrosion product.
Further, it appears that the hydrogen gas, etc. generated at the cathode in this corrosive reaction produces a swelling effect which continues as the coating layer gradually embrittles, eventually to flake off from the Fe-Al-Zn ternary alloy layer.
On the basis of their observations, the inventors directed their efforts toward attaining the three items (1)-(3) mentioned below with a view to preventing the flaking of the coating layer of a galvanized steel sheet with a zero-spangle during long-term indoor aging and, as a result, accomplished the present invention:
(1) To make possible the selective formation of coating spangles using the same coating bath; PA0 (2) To prevent generation of cracks in the coating layer even in the case where quenching treatment is carried out in order to make the spangle fine; and PA0 (3) To inhibit the corrosive reaction of Al segregated in the grain boundary or liberated in the coating layer.